I nearly shunted into the back of a laguna yesterday morning in exactly the same situation, dozy driver wasn't for setting off even with an empty roundabout but they edged forward a bit and while I was looking for oncoming cars I saw the movement out of the corner of my eye and took that to mean they had set off. Pressed the gas pedal as I turned around to look back at them, they'd only gone about 6 inches and stopped again! Brakes applied, stopped fairly short of them but it still had my heart pounding pretty hard!